A thermal processing chamber refers to a device that uses energy, such as radiative energy, to heat objects, such as semiconductor wafers. Such devices typically include a substrate holder for holding a semiconductor wafer and a light source that emits light energy for heating the wafer. For monitoring the temperature of the semiconductor wafer during heat treatment, thermal processing chambers also typically include radiation sensing devices, such as pyrometers, that sense the radiation being emitted by the semiconductor wafer at a selected wavelength. By sensing the thermal radiation being emitted by the wafer, the temperature of the wafer can be calculated with reasonable accuracy.
One major problem in the design of rapid thermal processing chambers having an optical temperature measurement system, however, has been the ability to prevent unwanted light radiated by the heater lamps from being detected by the pyrometric instrumentation. Should unwanted light not being emitted by the semiconductor wafer be detected by the pyrometer, the calculated temperature of the wafer may unreasonably deviate from the actual or true temperature of the wafer.
In the past, various methods have been used to prevent unwanted thermal radiation from being detected by the pyrometer. For instance, physical barriers have been used before to isolate and prevent unwanted light being emitted by the heater lamps from coming into contact with the pyrometer. Physical barriers have been especially used in rapid thermal processing chambers in which the heater lamps are positioned on one side of the semiconductor wafer and the pyrometer is positioned on the opposite side of the wafer.
Physical barriers, however, can restrict the system design. For instance, the physical barrier can restrict how the wafer is supported. In one embodiment, a light-tight enclosure is created below the wafer using a large diameter continuous support ring to hold the wafer at it edges. When a support ring is present, there can be overlap between the support ring and the edges of the wafer, which can lead to temperature non-uniformities in the wafer during heating cycles. Another problem can arise if the support ring or the wafer is warped even slightly. When this occurs, light can stray through the gap into the supposedly light-tight region. The stray light can induce errors in the pyrometer readings.
Besides physical barriers, spectral filters have also been used to limit the amount of light interference detected by the pyrometers. For instance, spectral filters can operate by removing light being emitted by the heater lamps at the wavelength at which the pyrometer operates. Preferably, spectral filters absorb unwanted thermal radiation while at the same time being transparent to the thermal radiation being emitted by the heater lamps that is necessary to heat the semiconductor wafer.
One type of spectral filter that has been used in the past is a window made from fused silica, such as silica doped with hydroxy (OH) ions. Fused silica glass is transparent to most light energy but is known to have several strong absorbing regions that are maximized at wavelengths of about 2.7 microns, 4.5 microns and at wavelengths equal to and greater than 5 microns.
Because certain OH-doped silica glass can effectively absorb light at wavelengths of 2.7, 4.5 and greater than 5 microns and is substantially transparent at many other smaller wavelengths of light, silica glass makes an effective spectral filter when the pyrometer contained within the thermal processing chamber is configured to sense thermal radiation at one of the above wavelengths.
Silica glass, however, is unfortunately not well suited to being used as a spectral filter in temperature measurement systems that contain pyrometers that sense thermal radiation at shorter wavelengths, such as less than about one micron. Specifically, in some applications, it is more advantageous and beneficial to operate pyrometers at relatively short wavelengths. In particular, by using pyrometers that operate at shorter wavelengths, the effects of wafer emissivity variations can be minimized providing for more accurate temperature determinations. Specifically, at lower wavelengths, silicon wafers are more opaque and the emissivity of the wafer is not significantly temperature dependent. The emissivity of the wafer is one variable that must be known with some accuracy in determining the temperature of wafers using pyrometers.
In addition to more precisely determining the temperature of wafers, pyrometers that operate at relatively shorter wavelengths are also generally less expensive and less complicated then pyrometers that are configured to operate at higher wavelengths. Further, pyrometers that sense thermal radiation at lower wavelengths generally operate very efficiently and can generate low noise measurements.
In the past, however, pyrometers that operate at lower wavelengths have been selectively used in thermal processing chambers due to the significant amount of stray light that can be detected in thermal processing chambers at lower wavelengths. As such, a need currently exists for a spectral filter that can efficiently absorb light energy at lower wavelengths, such as wavelengths less than about 2 microns.